Remember the Ozone hole? Decades ago it was a big concern. It was getting bigger and bigger and our emissions of ozone-depleting substances was identified as the main reason. It continues to get smaller as anthropogenic emissions continue to be reduced. It was slightly smaller in 2013 than average in recent decades, according to NASA satellite data.

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The ozone hole is a seasonal phenomenon that starts to form during the Antarctic spring (August and September). The September-October 2013 average size of the hole was 8.1 million square miles (21 million square kilometers). For comparison, the average size measured since the mid-1990s when the annual maximum size stopped growing is 8.7 million square miles (22.5 million square kilometers). However, the size of the hole in any particular year is not enough information for scientists to determine whether a healing of the hole has begun."There was a lot of Antarctic ozone depletion in 2013, but because of above average temperatures in the Antarctic lower stratosphere, the ozone hole was a bit below average compared to ozone holes observed since 1990," said Paul Newman, an atmospheric scientist and ozone expert at NASA's Goddard Space Flight Center in Greenbelt, Md.

The ozone hole forms when the sun begins rising again after several months of winter darkness. Polar-circling winds keep cold air trapped above the continent, and sunlight-sparked reactions involving ice clouds and chlorine from manmade chemicals begin eating away at the ozone. Most years, the conditions for ozone depletion ease before early December when the seasonal hole closes.

Levels of most ozone-depleting chemicals in the atmosphere have gradually declined as the result of the 1987 Montreal Protocol, an international treaty to protect the ozone layer by phasing out production of ozone-depleting chemicals. As a result, the size of the hole has stabilized, with variation from year to year driven by changing meteorological conditions.

The single-day maximum area this year was reached on Sept. 16 when the maximum area reached 9.3 million square miles (24 million square kilometers), about equal to the size of North America. The largest single-day ozone hole since the mid-1990s was 11.5 million square miles (29.9 million square kilometers) on Sept. 9, 2000.

The Antarctic ozone hole reached its maximum single-day area for 2013 on Sept. 16. The ozone hole (purple and blue) is the region over Antarctica with total ozone at or below 220 Dobson units (a common unit for measuring ozone concentration).